1. Field of the Invention
The present invention relates to a process for production of crystalline L-aspartic acid from fumaric acid with aspartase. The present invention also relates to a method for recycling the non-crystallized L-aspartic acid remaining in the mother liquor given after isolating the crystalline L-aspartic acid, by adding to the starting solution for the subsequent reaction.
2. Description of the Prior Art
A process for crystallizing and collecting a DL-aspartic acid has been disclosed in Japanese Patent Application Laid-open No. 48-56618 in which fumaric acid is added to a disodium DL-aspartate solution. This process comprises reacting disodium fumarate with a large excess of ammonia to form a DL-aspartic acid, expelling the excess ammonia from the reaction solution, and then adding fumaric acid to the reaction solution to crystallize and isolate a DL-aspartic acid.
In this process, the solution before the addition of fumaric acid is a solution of sodium DL-aspartate, to which fumaric acid is added to precipitate a DL-aspartic acid. The mother liquor (i.e., the filtrate) after the isolation of crystalline DL-aspartic acid is a solution of disodium fumarate. Subsequently, a large excess amount of ammonia based on the amount of fumaric acid is added to the disodium fumarate solution, which is recycled in turn as the starting solution for the subsequent reaction.
In general, in the reaction for producing an L-aspartic acid from ammonium fumarate with enzyme, addition of ammonia in an amount equimolar with the amount of the starting material (i.e., fumaric acid) or more is required. For shifting the equilibrium of the reaction to the L-aspartic acid side, ammonia is usually used in an amount of from 2 to 2.3 times the molar amount of fumaric acid. On the other hand, aspartase, the enzyme catalyzing the reaction, has the optimum pH of about 8.3. In a pH range too much higher than this pH value, the enzymatic activity is likely to be reduced disadvantageously.
In the process disclosed in Japanese Patent Application Laid-open No. 48-56618, a large excess amount of ammonia is added to the disodium fumarate solution for recycling.
The disodium fumarate solution has a pH of 8.4. When ammonia is added to this solution in the equimolar amount with the amount of fumaric acid, the pH of the solution rises to 12.1 at 30.degree. C., at which pH aspartase is denaturated. Therefore, the addition of such amount of ammonia is not suitable for the enzymatic reaction with aspartase.
Japanese Patent No. 2524306 has disclosed a process for crystallization and isolation of L-aspartic acid by adding fumaric acid to a monoammonium L-aspartate solution. In this process, a solution of diammonium fumarate is converted into a solution of monoammonium L-aspartate with aspartase, fumaric acid is added to the resultant solution to crystallize L-aspartic acid, which is isolated by filtration, and then ammonia is added to the filtrate for the subsequent reaction.
In this process, the salt exchange reaction between L-aspartic acid and fumaric acid is conducted under a heterogeneous condition where crystals of either fumaric acid or L-aspartic acid or both are present, since fumaric acid is added to the monoammonium L-aspartate solution.
Furthermore, in this process, the amount of fumaric acid first added is 0.5 times the molar amount of the L-aspartate present in the reaction solution. However, for recycling the reaction solution after isolating crystalline L-aspartic acid as the starting solution for the subsequent reaction, an additional amount of fumaric acid must be supplemented to the reaction solution to re-adjust the reduced substrate concentration to the initial one. That is, addition of a solid component (i.e., fumaric acid) is conducted twice. Increase in the number of such a complicated step of handling a solid component in a process may result in a poor workability and a poor purity of the L-aspartic acid yielded.